We demonstrate that the reflection spectrum of a fiber Fabry–PÉrot formed by a single-mode fiber and an external mirror does not exhibit the standard Lorentzian profile of a conventional bulk-optic Fabry-PÉrot, but is instead highly asymmetric. Measurements indicate that the sign of this asymmetry is different for dielectric and metal mirrors. We show that this asymmetry is due to one of two mechanisms, namely the beam diffraction in the cavity and the complex phase upon reflection from a metal layer. We present an analytical approach to accurately model these spectra, and provide simple analytical formulas, useful in the design and optimization of fiber Fabry-PÉrot-based sensors. Specifically, we present expressions for the maximum finesse and the condition to obtain full contrast in a fiber Fabry-PÉrot. These results are widely applicable, in particular to both low- and high-finesse interferometers.